Polyester resins are widely used as materials, e.g., for fibers, textiles, resins for molding and beverage bottles. In order to bring out molding processability and mechanical properties necessary for various applications, there is a technique of raising a polycondensation degree of the polyester resin (which is usually expressed by a molecular weight or an intrinsic viscosity) to a predetermined level. A known method therefor is, for example, a method of obtaining a prepolymer of polyester resin (which will be sometimes abbreviated hereinafter as a “polyester prepolymer”) by melt polycondensation, particulating the prepolymer, and raising the polycondensation degree by solid-phase polycondensation. Such a method is widely used in production of polyester resin particles on an industrial scale.
In this method, it is common practice to carry out a crystallization step and a drying step to suppress fusion and hydrolysis of particles, before subjecting the polyester prepolymer particles to the solid-phase polycondensation step. Since the polyester prepolymer particles might undergo fusion to bond to each other in the crystallization step and the drying step, various improvement methods have been proposed to avoid such fusion. For example, one of the proposals is a method in which polyester prepolymer particles are immersed in warm water at a temperature at least 10° C. lower than the glass transition temperature (Tg) thereof and in which the temperature is raised to a temperature 15° C. higher than Tg at a rate of below 100° C. per hour with water running at or over a specific linear velocity, thereby avoiding the fusion (cf. Patent Document 1). Other proposed methods include a method for treating polyester prepolymer particles with heated steam at 110° C. or higher before the drying step (cf. Patent Document 2) and a method for treating polyester prepolymer particles with water or steam at a temperature of from 80° C. to 100° C. for from 5 minutes to 25 minutes (cf. Patent Document 3).
On the other hand, mainly for the purpose of improvement in quality of polyester resin and molded products thereof, there are also proposed methods of bringing polyester resin particles into contact with water to reduce oligomers, acetaldehyde, and so on (cf. Patent Documents 4, 5 and 6).
Moreover, by virtue of excellent properties such as mechanical strength and chemical stability, PET is widely used for many materials and products, e.g., fibers, textiles, resins for molding and beverage bottles. Required molecular weights (intrinsic viscosities) of PET differ depending on the applications, and are, for example, usually from 0.70 to 0.95 dL/g for bottles and usually from 0.95 to 1.20 dL/g for tire cords.
In order to bring out the molding processability and mechanical properties necessary for the above-mentioned applications, it is necessary to increase the molecular weight (intrinsic viscosity) to a predetermined level, and a technique therefor widely used on an industrial scale is a method in which a PET prepolymer obtained by melt polycondensation of raw materials for polyethylene terephthalate is sequentially subjected to solid-phase polycondensation to increase the molecular weight. The solid-phase polycondensation is usually conducted by heating the PET prepolymer obtained by the melt polycondensation, in an inert gas atmosphere or under a reduced pressure, but it takes a relatively long period of time before achievement of a desired molecular weight. Therefore, there are desires for a production method with better productivity. A method proposed therefor is one in which a PET prepolymer with a relatively low polymerization degree is obtained by melt polycondensation and in which the prepolymer is subjected to solid-phase polycondensation at a high temperature.
However, the problem of fusion among PET prepolymer particles is likely to arise during the high-temperature solid-phase polycondensation. Heretofore, no sufficient study has been, however, conducted on the fusion.
For example, Patent Document 7 discloses a heat treatment step using two or more continuous moving beds in the solid-phase polycondensation step. The technique disclosed in Patent Document 7 is to raise the temperature stepwise so as to avoid the fusion of particles in the solid-phase polycondensation of PET prepolymer particles with medium molecular weights in the intrinsic viscosity range of about from 0.5 to 0.65 dL/g, but the polycondensation reaction rate is not necessarily improved. Furthermore, a melt polycondensation equipment for obtaining the prepolymer with the medium molecular weights was more expensive than an equipment for obtaining a low-molecular weight prepolymer and the method was not necessarily satisfactory from the overall viewpoint.
Patent Document 1: JP-A-1-180309
Patent Document 2: JP-A-59-25815
Patent Document 3: British Patent 836,742
Patent Document 4: JP-A-7-233248
Patent Document 5: JP-A-7-286046
Patent Document 6: JP-A-2004-67964
Patent Document 7: U.S. Pat. No. 5,408,035